Japanese patent application publication No. H7-021347 proposes a method for identifying the size of an original document being scanned with a scanner or other image-reading device. More specifically, a facsimile machine or other image-transmitting device employing this method has a special pattern disposed at an image-reading position. When scanning an original document, the image-reading device of the image-transmitting device also reads this pattern as a background image to the original document. Subsequently, the image-transmitting device performs a pattern-detecting process to detect the pattern in the image data generated during the scanning operation and identifies the outline of the original document based on the results of this pattern-detection process. In this way, the image-transmitting device can identify the size of the original document with accuracy.